An equalization technique for equalizing a waveform distortion using an electric circuit has been conventionally, actively utilized for high-quality data transmission in fields of wired communication using a metallic cable or the like and wireless communication.
However, since application of the technique to optical communication has been confronted by various disadvantages, it has just been made practical. Specifically, as examples of the application of the electric equalizer to the optical communication system proposed so far, examples of a configuration of a feed-forward equalizer (FFE) based on a transversal filter are disclosed in the Patent Literatures 1 and 2 (see below). In addition, an example of a configuration of a decision feedback equalizer (DFE) based on a transversal filter is disclosed in Patent Literature 3. Further, an example of a configuration of a Viterbi equalizer using maximum likelihood sequence estimation (MLSE) is disclosed in the Patent Literature 4 (see below).    Patent Literature 1: Japanese Patent Application Laid-open No. H8-163027    Patent Literature 2: Japanese Patent Application Laid-open No. 2000-299263    Patent Literature 3: Japanese Patent Application Laid-open No. 2001-308750    Patent Literature 4: Japanese Patent Application Laid-open No. 2001-7765
These application examples are characterized by easily performing small-sized and high-rate waveform shaping using an electric equalization filter that performs waveform equalization in an electric region without using an optical equalization filter that performs waveform equalization in an optical region. A distorted signal which has been subjected to an opto-electric conversion is decoded using the electric equalizing filter. By doing so, it is intended to ensure a high signal quality even in a transmission system which suffers waveform dispersion and polarization mode dispersion inherent in an optical transmission path.
Accordingly, each of these application examples is basically an extension of a utilization method of the electric equalizer in the wired communication and the wireless communication, and not a proposal of an optical communication system actively using the technique.
Meanwhile, with a view of improving the signal quality of the optical transmission system, if the electric equalization filter is used only for expecting an effect of improving a waveform of a received signal, only advantages such as a reduction in size and a reduction in cost of an apparatus are exhibited.
Only with the advantages, however, the obtained signal quality is as same as the signal quality obtained when an optical equalizing unit is employed. Therefore, these advantages do not essentially, greatly contribute to an improvement in function of the entire apparatus.
The present invention has been achieved in light of these circumstances, and has an object to obtain an optical communication system with an improved function using a waveform improving effect of an electric equalizer. Specifically, it is an object of the present invention to provide an optical receiver and an optical transmission system that uses this optical receiver capable of increasing a transmission rate and improving frequency utilization efficiency without degrading receiving characteristics.